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Along with other new evolution methods such as:

Pancham -> Pangoro

Eevee -> Sylveon

Sligoo -> Goodra

They were all revealed by Gamefreak prior to the release of the games.
I couldn't really find a site where it specifically said they confirmed it but I found someone saying something that pretty much confirms it.

>Do we actually know you need to hold it upside down, or is Game Freak just laughing at us?